Stack gas analysis has traditionally been performed by drawing a sample through a small tube inserted in the stack, collecting and fixing the pollutant in a solution or on a solic by absorption or reaction, and using conductimetric, colorimetric, or photometric analysis to determine the concentration of the pollutant. Some of the techniques in current use are described in "Standards of Performance for New Stationary Sources," Federal Register, 36, 24890 (Dec. 23 1971), the Los Angeles Air Pollution Control District "Source Sampling Manual," (1963), and by Cooper and Rossano in "Source Testing for Air Pollution Control," McGraw-Hill, New York, N.Y., 1971. More recently, methods have been developed which provide continuous records of pollutant concentrations. A review of instrumentation for continuous SO.sub.2 monitoring has been compiled by Hollowell et al in Analytical Chemistry, 45, 63A (1973).
A gas sample withdrawn directly from a stack must usually be conditioned before passing to a continuous analyzer or a wet chemical sampling train. The conditioning entails removing condensable vapors, mists, solid particulates, and chemical species known to interfere with the analysis of the desired pollutant. A typical sample-conditioning procedure might involve heating the gas to maintain vapors above their dew point or cooling to condense and remove the vapors from the sample stream, filtering the sample to remove particulates, and bubbling the gas through a liquid solution which removes the undesired chemical species but allows the pollutant to pass through to the analyzer.
A. O'Keeffe of the National Environmental Research Center, EPA, proposed using a polymer tube as an in situ stack sampling interface around 1972. In this proposed method, a carrier gas would pass continuously through a polymer tube mounted in the stack, and the pollutant would permeate the tube wall from the stack into the carrier gas stream, which would then pass to a continuous ambient analyzer. This method has several potential advantages over traditional sample conditioning techniques: an average concentration across the stack can be measured, polymers can be used which do not pass interfering pollutants, vapors in the carrier gas stream should be well above their dew points, and particulate filters are not required.
However, there are disadvantages to this system for commercial use. In the first place, the permeability of the interface is a function of temperature. Thus, the temperature must be recorded and the readings of the gas analyzer corrected for variations in this variable. Secondly, the pollutant concentration as measured by the gas analyzer is a function of the carrier gas flow rate. Thus, the flow rate must be kept constant or, alternatively, measured and the gas analyzer readings corrected for fluctuations in this variable.
It is the purpose of the present invention to provide a system which eliminates the disadvantages associated with current methods of stack gas monitoring and provides a continuous gas pollutant monitor having commercial attributes. It incorporates the idea of using a permeable interface for in situ stack sampling proposed by O'Keeffe. The inventors are the only ones who have developed and demonstrated the feasibility of the technique. The concept of the interface was described in Environmental Science and Technology, 7 545 (1973) and 10, 457 (1976). The complete apparatus, and most particularly the automatic temperature and flow rate compensation which are integral to the commercial viability of the device, is the subject of the current invention.
The only related invention disclosed by a patent search is a gas dilution apparatus, U.S. Pat. Nos. 3,833,016 (Sept. 3, 1974), assigned to Meloy Laboratories, Inc., of Springfield, Va. Like the present invention, the Meloy device uses a membrane to dilute a stack gas prior to analysis. However, the Meloy device is an out-of-stack sampler: a gas sample is withdrawn from the stack and contacts the membrane externally in a heated chamber. Contamination due to condensation, pollutant sorption, and particulate buildup--the problems the present invention is designed to overcome--are not eliminated by the Meloy device. The in situ method of calibration and operation, the cross-section averaged sampling, the flexibility in handling a wide range of stack pollutant levels and temperatures, and the data logging and analysis features of the present invention provide substantial advances in the state of the art over the Meloy apparatus.